Depending on what press you’ll print, you will notice that the grippers (Greifer) will need some space to hold the paper. It varies from one press to another. On my Vandercook Uni I, it’s 12.5 mm; on my Korrex Hannover, it’s 10 mm and on my Korrex Stuttgart, it’s 5 mm. Once you’ve established that, you take that into account when you build your forme, or you add extra to your paper.